1. Field of the Invention
The present invention relates to a process and an apparatus for making a substrate from a monolithic silicon wafer, and more particularly a substrate of a discrete element for use as a transistor, diode or the like.
2. Description of the Prior Art
Conventionally, it is known that such a substrate which is made from a single crystal Si-wafer (silicon wafer) has been used for various types of discrete elements such as a transistor, a pn-junction diode or the like.
As shown schematically in FIG. 9 to FIG. 11, a wafer 1 is sliced from a Si-ingot by a slicing saw having a blade core bonded with abrasive grains such as an industrial diamond or the like, and is then transferred to a thermal diffusion furnace for doping an impurity diffused layer 2 on both sides. The impurity layer may be composed of P(phosphorus), B(boron) or the like as the impurity compositions.
As a technical problem in the above doping process, however, it is noted that the above wafer 1 having the impurity diffused layers 2 on both sides is returned to a grinding process positioned upstream, and one side is ground by a grinding machine for removing that side's impurity layer in order to obtain a plain surface 3 without any impurity diffused layer. This feature is shown schematically in FIG. 10 wherein the reference letter G shows a grinding machine. According to this grinding process for a single side, a further impurity diffused layer 4 is doped on the plain surface 3 once again to provide a transistor, such as bipolar transistor which is doped with an emitter and base through those preparatory processes, such as etching, polishing, or the like, which are positioned upstream. After the completion of doping any new impurity 4 in the plain surface 3, the wafer is further transferred to a wiring station having a die. The above feature is shown schematically in FIG. 11.
In the above conventional process, since it is required to again grind on side of the wafer 1 to provide a wafer having both sides, that is, the plain surface 3 and the old surface 2 having an impurity diffused layer 2 on both sides, the problem is obvious that valuable silicon material is wasted as a grinding loss as well as the impurity diffused layer having to be doped a second time.
According to the prior art, Japanese Provisional Patent Application No. 64-30715 discloses a silicon block which consists of a plurality of silicon wafers stacked and bonded together in the shape of a Si-ingot, which is sliced by an ID (inner diameter) saw gradually at a predetermined thickness.
In this prior art device, however, despite the wafer being sliced with a predetermined thickness, the sliced wafer is formed as two pieces of the wafer and is bonded by an adequate bond but each are not a solid wafer. In addition, prior to the slicing process in this document, the wafer does not have any impurity diffusion layers in both sides. Therefore, it is clear that this document teaches a process for making a substrate of a pn-junction type element with a sintered layer sandwiched between two pieces of different kinds of wafers.
Accordingly, Japanese Provisional Patent Application No. 64-30715 does not discloses a process for making a substrate having an impurity diffused layer on one side and a plain surface without any impurity diffused layer on the other side of a Si-wafer, and also does not disclose a means for obtaining two pieces of substrates from one piece of a Si-wafer and also a means to re-slice a wafer at the center portion of its core thickness. These features will be described below as characteristics of the present invention.
In a comparison between both processes, Japanese Provisional Patent Application No. 64-30715 can obtain only one piece of a substrate for a pn-junction element from two pieces of a wafer as well as two substrates of pn-junction element from three pieces of a wafer.
In contrast, the present invention can obtain four pieces of a substrate of a pn-junction element from three pieces of a wafer. Thus, the technical objects and advantages of the present invention are quite different from the above prior art.